This invention concerns rectifier units for producing direct current from a multiphase a.c. source.
Circumstances arise where it is desired to obtain a relatively smooth d.c. output from a multiphase, usually 3-phase, a.c. supply. In such circumstances it is known to use what is called 12-pulse rectifier operation, and this can be achieved by the use of single-way phase-displaced transformer winding groups, or by using two phase-displaced 3-phase rectifier bridge circuits connected either in parallel or in series.
This invention is applicable to rectifier units with two phase-displaced multiphase rectifier bridge circuits connected in parallel. Such units may for example be used as sub-station rectifiers for railway traction.
A simple 3-phase bridge rectifier, if connected to a 3-phase a.c. source, will supply an output consisting, in each cycle of the alternating current, of the sum of six unidirectional sinusoidal pulses each a third of a cycle long and all being phase displaced by intervals of sixty degrees. This may be termed 6-pulse operation.
If two such bridge rectifiers are arranged to be supplied from two 3-phase a.c. sources which differ in phase by 30 degrees, or an odd integral multiple thereof, the two unidirectional outputs of the bridge rectifiers will each be of six-pulse type, but the pulses from one rectifier will lie intermediate the pulses from the other. Thus, if the two outputs are combined, twelve-pulse operation will result, with a consequent reduction in the amount of ripple on the final output.
With such a rectifier unit it is not easy to achieve equal current sharing between the two bridges. With this in mind several factors have to be looked after carefully, all tending to add to the cost, otherwise there could be a current imbalance as bad as three to one. The balance may in any event be upset by supply voltage harmonics.